Voltage regulators as a part of direct current power supplies are a ubiquitous, if often unseen part of modern life. Almost all electronic devices contain a regulated power supply. Semiconductor devices generally operate at a relatively low direct current voltage, for example 5 volts. Much of the electrical energy to power electronic devices is made available at different voltages. For example, mains power in the United States is nominally 120 volts AC. Automotive power is nominally 12 or 24 volts DC, but is subject to high voltage transients, for example 60 volts, during engine start and other conditions of changing loads.
Power supplies are generally employed to match the requirements of electronic devices (and other types of machines) to the available conditions of electrical power. Many devices, for example hand held electronics, powered by batteries nominally within the voltage range of the electronics employ power supplies to compensate for non-linear discharge characteristics of batteries and to extract as much energy from the batteries as possible.
An important part of most power supplies is a voltage regulator. Voltage regulators function to maintain voltage (and/or current) within a range of output values, for example five volts plus or minus two percent (5 v+/−2%). It is generally important to maintain an output voltage within the specified range. Too high a voltage may damage semiconductor devices, leading to decreased reliability or outright failure. If the voltage goes too low, voltage compliance is lost on many components which may lead to several types of failure. In addition, changes in power supply voltage may induce noise into subsequent processing.
An important part of most voltage regulators is a voltage reference. A voltage reference provides a reference voltage that is compared against the output of the voltage regulator. Circuitry within the voltage regulator adjusts the output of the voltage regulator to have a desirable relationship to the voltage reference.
A “bandgap” is generally understood to refer to or to describe the energy difference between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. Bandgaps are a well known source of reference voltages within integrated circuits.
In order to accommodate a voltage regulator having a variety of output voltages, e.g., 1.8 volts, 3.3 volts, 5 volts, etc., it is desirable to create a bandgap voltage reference based upon a minimum bandgap voltage. For silicon-based integrated circuits, this minimum bandgap voltage is generally 1.25 volts. In addition, a very high power supply rejection ratio is desirable as such rejection ratio affects the size of a required compensation capacitor (both in terms of capacitance and physical size) required on the regulator output. Generally, a higher power supply rejection ratio enables a smaller output capacitor. Unfortunately, conventional bandgap voltage reference designs offer less than desirable power supply rejection ratios, comprise too high a reference voltage, require an unfavorably large integrated circuit area and/or require undesirably large output filtering capacitors.
Accordingly, it is desirable to provide a system and method for a bandgap reference. A further desire exists for providing a bandgap reference with a high power supply rejection ratio and a favorably small integrated circuit area while requiring a small output filtering capacitor. A still further desire exists for the above-mentioned capabilities to be achieved in a manner that is complimentary and compatible with standard semiconductor processes.